EL James, the author behind the erotic trilogy and, according to Forbes, the richest writer in the world, broke the news on her Twitter feed on Monday. Ms James wrote: "I am delighted to let you know that the lovely Dakota Johnson has agreed to be our Anastasia in the film adaptation of Fifty Shades of Grey." An hour later, she posted: "The gorgeous and talented Charlie Hunnam will be Christian Grey in the film adaptation of Fifty Shades of Grey."

Johnson, who is 23, is the daughter of actors Don Johnson and Melanie Griffith. She has appeared on screen with Griffith in Crazy in Alabama, her film debut in 1999, but is better known for supporting roles in rom-com The Five Year Engagement and Facebook film, The Social Network, in which she appeared opposite Justin Timberlake.

Newcastle-born Hunnam, who is 33, came to fame after appearing in popular North East TV series, Byker Grove. He has since appeared in Cold Mountain in 2003, Children of Men and, most recently, starred in Guillermo Del Toro's Pacific Rim as the retired fighter pilot Raleigh Becket.

Ever since film interest in making the best-selling trilogy was made public in March 2012, Fifty Shades fans have been guessing which actors would be cast in the picture. Previously, Emma Watson, Mila Kunis and Elizabeth Olson have all denied any association with the role of Anastasia, or Ana, the naïve university student who gets into a relationship with the older and domineering millionaire, Christian Grey.

Hunman was among the few contenders thought to be connected to the role of Grey, along with Michael Fassbender, Robert Pattinson and Ian Somerhalder.